Recently, studies for application of plasma generated under atmospheric pressure conditions to medical operations have become active, and development of plasma technologies as medical equipments has been accelerated worldwide. However, introduction of plasma technologies has been being promoted without measurement of characteristics of the plasma to be used, and evaluation of safety of the plasma. In fact, when the energy of the plasma is high, tissues may be damaged.
As characteristics of plasma, electron temperature, electron density, ion temperature, ion density, and space potential have been measured in, particularly, vacuum. Meanwhile, as for non-equilibrium low-temperature plasma under an atmospheric pressure, measurements such as emission spectrometric measurement and electron temperature measurement have just been started. Among the parameters to be measured, the level of the current value to flow through a living body (a human body) is the most influential to the safety. Therefore, it is necessary to measure a current to actually flow, and to control and adjust the plasma treatment equipment.
As common methods for measuring a current, a method of incorporating resistors on a current path, measuring a voltage across the resistors, and evaluating a current based on the resistance values and the voltage value, a method using a Rogowski coil, and a method using a current transformer have been discovered and used traditionally. However, it has not been easy to measure a current of a plasma jet generated by a plasma generation equipment under atmospheric pressure conditions or a current flowing through a living body. The reason is as follows. To generate a non-equilibrium low-temperature plasma under an atmospheric pressure, it is common to fluctuate a plasma generation electrode over time. The current of an electrode of the plasma generation equipment temporally fluctuates at a phase difference from the voltage of the electrode. The current flowing between the voltage application electrode and a ground electrode of the plasma equipment is joined by a current flowing through a living body. The current flowing through the living body has a pulsed shape fluctuating temporally sharply, and there is also a displacement current component to join them, which makes it likely for the voltage applied to the electrode to fluctuate according to the plasma state, which consequently results in fluctuation of the plasma state. In this way, the plasma state, the voltage across the electrodes of the plasma generation equipment, the current between the electrodes of the plasma generation equipment, and the current flowing through the living body are related with each other. Even though it has been possible to measure the current of the supply-side power source of the plasma generation equipment, it has not been easy to separate the current flowing through the living body therefrom, because the current is superposed with the current between the electrodes of the plasma generation equipment.
Hence, the problems are the inability to easily measure and evaluate plasma characteristics of medical plasma treatment equipments expected to spread at home and abroad, and unavailability, particularly at the medical front, of a user-friendly system taking into consideration operability, hygiene, and compactness. That is, there has not been an evaluation system that can maintain and manage the initial performance of the plasma equipments for safety and security, and in addition, it has not been possible to perform a plasma treatment while easily monitoring (measuring or evaluating) the plasma state.
As such, a system and a method for easily measuring and objectively evaluating the state of plasma generated by a plasma treatment equipment, such as those of PTL 1 and NPL 1, under atmospheric pressure conditions have been strongly demanded.
Meanwhile, PTL 2 describes a plasma treatment equipment that, with a photon detecting sensor for measurement of an ultraviolet-induced current provided on a wafer stage, detects an abnormal electrical discharge phenomenon to occur in the plasma chamber real-time simultaneously with an ongoing semiconductor wafer treatment. However, this equipment is an industrial plasma treatment equipment for wafer fabrication, etc. configured to measure a photoelectron current attributable to an abnormal electrical discharge to occur in a region different from a region for a main electrical discharge, and not configured to measure a current to flow through a treatment target of medical plasma treatment equipments, particularly, a living body, etc. Therefore, this equipment can neither be applied to medical purposes, nor used conveniently at the medical front.